Abstract

X-ray Raman spectroscopy (XRS) combined with small-angle x-ray scattering (SAXS) were used to study aqueous solutions of HCl and NaOH. Hydrated structures of H+ and OH− are not simple mirror images of each other. While both ions have been shown to strengthen local hydrogen bonds in the hydration shell as indicated by XRS, SAXS suggests that H+ and OH− have qualitatively different long-range effects. The SAXS structure factor of HCl (aq) closely resembles that of pure water, while NaOH (aq) behaves similar to NaF (aq). We propose that protons only locally enhance hydrogen bonds while hydroxide ions induce tetrahedrality in the overall hydrogen bond network of water.

This work was supported by the (U.S.) National Science Foundation (NSF) CHE-0809324, the Department of Energy through the SLAC Laboratory Directed Research and Development Program and the Swedish Research Council. Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a national user facility operated by Stanford University on behalf of the (U.S.) Department of Energy (DOE), Office of Basic Energy Sciences. The BioSAXS beamline 4-2 and the SSRL Structural Molecular Biology Program are supported by the Department of Energy, Office of Biological and Environmental Research, and by the National Institutes of Health (NIH), National Center for Research Resources, Biomedical Technology Program.